A closed line consisting of all points on the plane that are equidistant from a given point on the plane is called a circle.
  • In the definition, the given point on the plane is called the center of the circle. Usually, we denote the center by the letter \(O\).
  • The circle is called the circle along with the part of the plane it delimits.
To draw a circle, take a compass with a pen/pencil attached to it and place the sharp end of the compass on a particular point of a paper, then keeping the abrupt end non-movable rotate the compass \(360\) degrees, so the pen/pencil draws a perfect circle.
The circle, in the drawing, is black. The black line, together with the shaded area forms a circle. Where, \(O\) is the center of the circle.
  • The segment joining the center to a freely chosen point on the circumference is called the radius. We usually denote radius by the lowercase \(r\) or the uppercase \(R\).
  • The  segment that joins the two points of the circle is called a chord.
  • The chord passing through the center of the circle is called the diameter.
d=2R or R=d2=0.5d, where \(d \) is the diameter of the circle and \(R \) is the radius of the circle.
\(AO\)\(=\)\(BO\)\(=\)\(EO\)\(=\)\(FO\) as circular radii.
The radii \(EO\) and \(FO\) form the diameter \(EF\).
\(BC\) is a chord.